Drying assembly

ABSTRACT

A drying assembly is disclosed. An example apparatus includes fans; a temperature sensor; a print engine to access instructions to cause a printer to perform a printing operation on media; and a processor responsive to an output of the temperature sensor to control the fans to force air onto the media to cause a substantially uniform temperature to be maintained across a width of the media during the printing operation and to cause a sum of air flow output by the fans to be maintained at a substantially constant value.

BACKGROUND

Many printers use liquid inks to print images onto media. Some of theliquid inks need to be evenly cured across the page to ensure properdurability and even gloss in the printed output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an example printer.

FIG. 2A is block diagram of an example drying assembly.

FIG. 2B is an isometric view of an example drying assembly.

FIG. 3 is a block diagram of an example printer.

FIG. 4 is an example block diagram of the processor coupled to memory.

FIG. 5 is a flow chart for an example method for controlling the fans ina drying assembly.

DETAILED DESCRIPTION

FIG. 1 is a side view of an example printer 100. The printer comprisesmedia supply system 102, media 104, inkjet print bar 106 and dryingassembly 108. In this example media 104 is a continuous sheet suppliedby media supply system 102. In other examples media may compriseindividual sheets. Media 104 is fed from media supply system 102underneath print bar 106. Inkjet heads on print bar 106 deposit ink ontomedia 104. In other example printers, there may be an intermediatetransfer blanket that receives ink from the inkjet heads and transfersthe ink to the media. Once the ink has been deposited onto the media,the media passes underneath the drying assembly 108. Drying assembly 108forces heated air past media 104 as shown by arrow 110. The heated airdries and cures the ink deposited onto the media. Print bar 106 may alsodeposit additional compounds onto media, for example gloss coats and thelike.

FIG. 2A is a block diagram of drying assembly 108. Drying assemblycomprises N fan units, where N is an integer greater than 1. Each fanunit comprises a fan housing 212, a fan 214, a heating element 216 and atemperature sensor 218. The fan units are attached to support 220 in aspaced apart relationship. Each fan 214 is located inside a fan housing212 and forces air in the direction shown by arrow 110. The heatingelements 216 may also be located inside the fan housings 212. Theheating elements 216 heat the air moved by the fans 214. The temperaturesensors 218 are located near the fan exhaust and can monitor thetemperature of the air as it leaves each fan housing 212.

FIG. 2B is an isometric view of drying assembly 108. In this examplethere are 4 fan units spaced along support 220. The fan units are spacedapart by distance X, where distance X is 425.6 mm. In other examplesthere may be a different number of fan units, for example three fanunits spaced apart by 487 mm.

The speed of each fan can be controlled independently. The fan speedsare adjusted with a fan speed control signal, typically a pulse widthmodulation (PWM) signal. The temperatures of the heating elements arecontrolled with a heating element control signal. In one example asingle heating element control signal is used for all of the heatingelements. Typically each of the N heating elements may have someresistance variability. In addition each of the N fans may run at aslightly different speed given the same input signal. Due to thesevariations, the air temperature exiting each fan may be different evenwith the same input control signals (i.e. the fan speed control signaland the heating element control signal). The variation in airtemperature can cause uneven curing and drying across the page.

In one example, a controller reads each temperature sensor to determinethe air temperature at each fan exhaust. The controller adjusts thespeed of each fan based on the air temperature to maintain the same airtemperature at each fan exhaust. The controller also maintains the totalair flow through all the fans as a constant value. One way to keep thetotal airflow constant is to keep the sum of the PWM from all of thefans at a constant value. In one example, all the heating elements willbe coupled together and controlled using a single heating elementcontrol signal. Using this method the temperature uniformity across thepage can be maintained and de-coupled with the power control of theheating elements.

FIG. 3 is a block diagram of an example printer. Printer comprises aprocessor 330, memory 332, input/output (I/O) module 334, print engine336 and controller 338 all coupled together on bus 340. In some examplesprinter may also have a display, a user interface module, an inputdevice, and the like, but these items are not shown for clarity.Processor 330 may comprise a central processing unit (CPU), amicro-processor, an application specific integrated circuit (ASIC), or acombination of these devices. Memory 332 may comprise volatile memory,non-volatile memory, and a storage device. Memory 332 is anon-transitory computer readable medium. Examples of non-volatile memoryinclude, but are not limited to, electrically erasable programmable readonly memory (EEPROM) and read only memory (ROM). Examples of volatilememory include, but are not limited to, static random access memory(SRAM), and dynamic random access memory (DRAM). Examples of storagedevices include, but are not limited to, hard disk drives, compact discdrives, digital versatile disc drives, optical drives, and flash memorydevices.

I/O module 334 is used to couple printer to other devices, for examplethe Internet or a computer. Print engine 336 may comprise a media supplysystem, a printhead, a drying assembly, an ink supply system, and thelike. Printer has code, typically called firmware, stored in the memory332. The firmware is stored as computer readable instructions in thenon-transitory computer readable medium (i.e. the memory 332). Processor330 generally retrieves and executes the instructions stored in thenon-transitory computer-readable medium to operate the printer. In oneexample, processor executes code that directs controller 338 to controla drying assembly in the print engine 336.

FIG. 4 is an example block diagram of the processor 330 coupled tomemory 332. Memory 332 contains firmware 442. Firmware 442 contains adrying module 444. The processor 330 executes the code in drying module444 to direct controller 338 to control the drying assembly 108.

Controller 338 is used to control the drying assembly 108. Dryingassembly 108 heats the ink, media and any other components deposited onthe media. The ink is heated to above a predetermined temperaturethreshold to ensure proper curing. The ink is also heated uniformlyacross the width of the media. In some examples two controllers may beused, one controller to control the fan speeds and thereby control thetemperature uniformity across the page, and one controller to controlthe power to the heating elements thereby controlling the averagetemperature of the air leaving the drying assembly. In other examplesone controller will be used to control both the fan speed and theheating elements. The single controller will still control the twosystems independently.

The controller adjusts the power to the heating elements and the speedof the fans to ensure that the ink reaches the threshold temperatureevenly across the media. In one example, all of the N heating elementsare coupled together and receive the same power setting. The controlleradjusts the power setting to the N heating elements to control theaverage temperature of the air leaving the drying assembly 108. Thecontroller can adjust the speed of each of the N fans 214 independently.The controller adjusts the fan speed of individual fans to maintain auniform temperature across the width of the media while keeping the sumof the air flow through all the fans constant. One way to keep the totalairflow constant is to keep the sum of the PWM from all of the fans at aconstant value.

FIG. 5 is a flow chart for an example method for controlling the fans ina drying assembly. The fan speed control method starts at step 550 wherethe startup parameters are set. The startup parameters include theinitial fan speed control signal for each of the N fans. The startupparameters may include a delay time to allow the fans to get up to speedbefore entering the fan speed control loop. Concurrently with the startof the fan speed control method, a temperature control method is alsostarted. The temperature control method is used to keep the averagetemperature exiting the fans at a given value.

After block 550 the fan speed control method proceeds to block 552.Block 552 is the start of the fan speed control loop. At block 552 theair temperature near the exhausts of each of the N fans is determined byreading the temperature sensors for each fan unit. At block 554 theaverage air temperature is calculated as well as a delta temperature ateach fan unit. The delta temperature for each fan unit is the averageair temperature minus the air temperature at that fan unit. In oneexample, at block 556 the delta air temperature for each fan unit iscompared to a threshold value. When all of the delta temperatures arebelow the threshold value the temperature uniformity across the fanunits is within a predetermined range. Therefore flow returns to block552.

When the delta temperature of any of the fan units is above thethreshold value, flow continues at block 558. In another example, thedelta air temperature for each fan unit is not compared to a thresholdvalue, flow automatically proceeds from block 554 to block 558. At block558 new fan speeds are calculated for each fan unit. A negative deltatemperature for a fan unit means the air temperature at that fan unit ishigher than the average air temperature. A positive delta temperaturefor a fan unit means the air temperature at that fan unit is lower thanthe average air temperature. The fan speeds for fans with airtemperature higher than the average air temperature (i.e. a negativedelta temperature) are increased. The fan speeds for fans with airtemperature lower than the average air temperature (i.e. a positivedelta temperature) are decreased.

The sum of the airflow through all the fans is kept at a constant value.One way to keep the total airflow constant is to keep the sum of the PWMfrom all of the fans set to a predetermined value. For example, whenthere are 4 fans, the sum of the PWM signals from each fan will be setequal to a predetermined value (predeterminedvalue=PWM1+PWM2+PWM3+PWM4). When the predetermined value is 200% thePWM's for the 4 fans may be 50%, 45%, 53% and 52% respectively. Thepredetermined value may be changed by the servo that controls theabsolute pressure in the chamber. Once the new fan speeds are calculatedthe fan speed control signals are updated with the new values. Flow thenreturns to block 552.

The fan speed control signal is typically a pulse width modulation (PWM)signal. In one example, equation 1 is used to determine the new fanspeed control signal at block 558.

PWM_(i)(t+Δt)=PWM_(i)(t)+K _(int)*err_int_i)(t+Δt)   Equation 1

Where PWM_(i)(t+Δt) is the new fan speed control signal at time t plusdelta time (Δt) for the i^(th) fan unit, PWM_(i)(t) is the old fan speedcontrol signal at time t for the i^(th) fan unit, K_(int) is the gainfor the interval delta time, and err_int_i(t+Δt) is the error signal forthe i^(th) fan unit for the interval delta time. Delta t (Δt) may be inthe range between 0.1 second through 40 seconds, for example 1 second.

In one example, K_(int) is calculated using equation 2.

K _(int)=0.04% PWM/C   Equation 2

Where % PWM/C is the relationship between the % PWM signal and thetemperature (Celsius). In other examples K_(int)=may be set in the rangebetween 0.5% PWM/C through 0.001% PWM/C.

In one example err_int_i(t+Δt) is determined using equation 3.

err_int_i(t+Δt)=1/Δt∫ _(t) ^(t+Δt)(T _(i) −T _(ave))dt[=]C   Equation 3

where T_(i) and T_(ave) are the air temperature at the i^(th) fan unitand the average air temperature respectively. By definition the sum ofthe error signals for all of the fan units is equal to zero. Thismaintains a total constant airflow across all the fan units.

In another example a derivative term is added to equation 1 to improvethe stability of the servo loop. The derivative takes into account therelative slope of the temperature (T_(i)) vs. time (t) curve at each fanunit compared to the average temperature (T_(ave)) vs. time (t) curve.Equation 1 becomes equation 4.

PWM_(i)(t+Δt)=PWM_(i)(t)+K _(int)*err_int_i(t+Δt)+K _(d)*err_der_i(t+Δt)  Equation 4

Where K_(d)=0.6% PWM/(C/sec) and err_der_i(t+Δt) is defined in equation5.

err_der_i(t+Δt)=1/Δt∫_(t) ^(t+Δt)({dot over (T)} _(i) −{dot over (T)}_(ave))dt[=]C/s   Equation 5

Where {dot over (T)}_(i) and {dot over (T)}_(ave) are the slope of thetemperature vs. time curve for the i^(th) fan unit and the temperaturevs. time curve for the average temperature, respectively.

The thermal gain of the system is defined as the change in airtemperature for a given change in the PWM percent (C/PWM %). In someexamples the thermal gain is between 4 and 15 degrees C. for a change ofone percent in the PWM duty cycle, for example 6.67 C/PWM %. Because ofthis thermal gain, small changes in the fan speed control signal cancause large changes in air temperature. During operation a typical rangefor the fan speed control signal is between 40%-90% PWM.

The change in air speed/pressure for a given change in PWM % in theaverage fans speed control signal is dependent on the number of fanunits, the fan type, the absolute PWM of the fan speed control signaland the fan outlet/exhaust geometry. In one example for a dryingassembly with three fan units, at an absolute fan speed control signalof 83% PWM (in all 3 fans) results in 2.3 m³/min (or a 4.6 mmH₂Opressure). For the same system, at an absolute fan speed control signalof 73% PWM (in all 3 fans) results in 2.0 m³/min (or a 3.8 mmH₂Opressure). Therefore the Pressure gain is (4.6−3.8)/10=0.08 mmH2O/PWM %and the Airflow Gain is (2.3−2.0)/10=0.03(m̂3/min)/PWM %. Duringoperation a typical air speed at the fan exhaust is between 5-20 m/sec.

An example drying assembly including a number N of fan units directed toforce air to a drying zone, where N is an integer 2 or greater and eachfan unit including a fan; a heating element positioned to heat the airmoved by the fan; and a temperature sensor positioned near an exhaust ofthe fan unit; a controller coupled to each fan unit, the controller tomonitor the temperature senor in each fan unit, the controller toindependently adjust a speed of each fan to maintain the sametemperature at all N fan units, the controller to keep the total airflowthrough all N fan units at a constant value.

In some examples, each of the heating elements in all N fan units arecoupled together and controlled with a single heating element controlsignal. In some examples, the speed of each fan is independentlyadjustable using a fan speed control signal, where each fan speedcontrol signal is a pulse width modulation (PWM) signal, and where anadjusted fan speed control signal for each fan is equal toPWM_(N)(t)+K_(int)*err_int_N(t+Δt), where PWM_(N)(t) is a fan speedcontrol signal at time t for the N^(th) fan unit, K_(int) is a gain forthe interval delta time (Δt), and err_int_N(t+Δt) is an error signal forthe N^(th) fan unit for the interval delta time (Δt). In some examples,the adjusted fan speed control signal for each fan includes the termK_(d)*err_der_N(t+Δt) where K_(d) is a gain and err_der_N(t+Δt) is anerror signal for the the N^(th) fan unit for the interval delta time(Δt) that is based on a relative slope of the temperature (T_(N)) vs.time (t) curve for the N^(th) fan unit compared to an averagetemperature (T_(ave)) vs. time (t) curve. In some examples, delta time(Δt) is in the range from 0.1 second to 40 seconds.

The examples, the printer includes the controller to determine anaverage temperature for all of the fans; the controller to determine adelta temperature for each fan where the delta temperature equals theaverage temperature minus a temperature at each fan; the controller tomaintain the same fan speed for each of the fans when the deltatemperature for all of the fans is below a threshold. In some examples,the N is in the range from 3 to 8. In some examples, the printerincludes a support wherein the fan units are spaced along the support bydistance X, where distance X is in a range from 30 mm to 800 mm.

An example method of controlling a drying assembly includes determiningthe temperature of air leaving each of N fan units where N is a integergreater than one; calculating an average air temperature for all N fans;decreasing a fan speed for each fan with an air temperatures lower thanthe average air temperature; increasing the fan speed for each fan withan air temperature higher than the average air temperature; maintaininga sum of the airflow through all N fans at a constant value. In someexamples, the method includes adjusting a heating element in each of theN fan units using a single servo control signal. In some examples, themethod includes increasing or decreasing the fan speed for each fan onceevery second. In some examples, there are 3 or 4 fan units.

In some examples, the fan speed is controlled using a pulse widthmodulation (PWM) signal, and where an adjusted fan speed control signalfor each fan is equal to PWM_(N)(t)+K_(int)*err_int_N(t+Δt), wherePWM_(N)(t) is a fan speed control signal at time t for the N^(th) fanunit, K_(int) is a gain for the interval delta time (Δt), anderr_int_N(t+Δt) is an error signal for the N^(th) fan unit for theinterval delta time (Δt). In some examples, the adjusted fan speedcontrol signal for each fan includes the term K_(d)*err_der_N(t+Δt)where K_(d) is a gain and err_der_N(t+Δt) is an error signal for the theN^(th) fan unit for the interval delta time (Δt) that is based on arelative slope of the temperature (T_(N)) vs. time (t) curve for theN^(th) fan unit compared to an average temperature (T_(ave)) vs. time(t) curve. In some examples, the sum of all of the PWM control signalsfor each fan is maintained at a predetermined value.

An example printer includes fan units to force air onto media during aprinting operation-each of the fan units including: a fan; a heater toheat air moved by the fan; and a temperature sensor positioned near anexhaust of the fan unit; and a controller to, in response to temperaturevalues obtained from the temperature sensors, at least one of (1)dynamically adjust at least one of the fans or (2) dynamically adjust atleast one of the heaters to maintain-a substantially uniform temperatureacross a width of the media during the printing operation-and tomaintain a sum of air flow exiting the fans to be at a substantiallyconstant value. In some examples, the controller includes a plurality ofcontrollers. In some examples, the printing operation includesdepositing printing fluid onto the media. In some examples, the heatersof the fan units are coupled together and controlled with a singleheating element control signal.

An example printer includes a number N of fan units directed to forceair to a drying zone, where N is an integer 2 or greater and each fanunit includes: a fan; a heating element positioned to heat air moved bythe fan; and a temperature sensor positioned near an exhaust of the fanunit; a controller coupled to each fan unit, the controller to monitorthe temperature sensor in each fan unit, the controller to independentlyadjust a speed of each fan to maintain a same temperature at all N fanunits, the controller to keep a total airflow through all N fan units ata constant value, wherein the speed of each fan is independentlyadjustable using a fan speed control signal, where each fan speedcontrol signal is a pulse width modulation (PWM) signal, and wherein anadjusted fan speed control signal for each fan is equal toPWM_(N)(t)+K_(int)*err_int_N(t+Δt), wherein PWM_(N)(t) is a fan speedcontrol signal at time t for the N^(th) fan unit, K_(int) is a gain forthe interval delta time (Δt), and err_int_N(t+Δt) is an error signal forthe N^(th) fan unit for the interval delta time (Δt).

In some examples, the adjusted fan speed control signal for each fanincludes the term K_(d)*err_der_N(t+Δt), where K_(d) is a gain anderr_der_N(t+Δt) is an error signal for the the N^(th) fan unit for theinterval delta time (Δt) that is base on a relative slope of thetemperature (T_(N)) vs. time (t) curve for the N^(th) fan unit comparedto an average temperature (T_(ave)) vs. time (t) curve. In someexamples, the delta time (Δt) is in the range from 0.1 second to 40seconds.

An example printer includes a number N of fan units directed to forceair to a drying zone, where N is an integer 2 or greater and each fanunit includes: a fan; a heating element positioned to heat air moved bythe fan; and a temperature sensor positioned near an exhaust of the fanunit; a controller coupled to each fan unit, the controller to monitorthe temperature senor in each fan unit, the controller to independentlyadjust a speed of each fan to maintain a same temperature at all N fanunits, the controller to keep a total airflow through all N fan units ata constant value, the controller to determine an average temperature forall of the fans; the controller to determine a delta temperature foreach fan where the delta temperature equals the average temperatureminus a temperature at each fan; the controller to maintain a same fanspeed for each of the fans when the delta temperature for all of thefans is below a threshold. In some examples, N is in a range from about3 to 8 fan units. In some examples, the printing operation includesdepositing printing fluid onto the media. In some examples, the printerincludes a support, wherein the fan units are spaced along the supportby distance X, where distance X is in a range from 30 mm to 800 mm.

An example method of controlling a drying assembly includes forcing aironto media during a printing operation using fan units of a printer, thefan units respectively including a fan, a heater, and a temperaturesensor; obtaining temperature values from the temperature sensors, thetemperature values representing a temperature of air exiting the fanunits; and dynamically adjusting at least one of (1) at least one of thefans or (2) at least one of the heaters to maintain a substantiallyuniform temperature across a width of the media during the printingoperation and to maintain a sum of air flow exiting the fans to be at asubstantially constant value. In some examples, the method includesincreasing or decreasing a fan speed for each fan within a thresholdtime period. In some examples, the printer includes 3 or 4 fan units.

An example of controlling a drying assembly includes determining atemperature of air leaving each of N fan units where N is an integergreater than one; calculating an average air temperature for all N fans;decreasing a fan speed for each fan with air temperatures lower than theaverage air temperature; increasing the fan speed for each fan with anair temperature higher than the average air temperature; maintaining asum of the airflow through all N fans at a constant value, wherein thefan speed is controlled using a pulse width modulation (PWM) signal, andadjusted fan speed control signal for each fan is equal toPWM_(N)(t)+K_(int)*err_int_N(t+Δt), where PWM_(N)(t) is a fan speedcontrol signal at time t for the N^(th) fan unit, K_(int) is a gain forthe interval delta time (Δt), and err_int_N(t+Δt) is an error signal forthe N^(th) fan unit for the interval delta time (Δt).

In some examples, the adjusted fan speed control signal for each fanincludes the term K_(d)*err_der_N(t+Δt), where K_(d) is a gain anderr_der_N(t+Δt) is an error signal for the the N^(th) fan unit for theinterval delta time (Δt) that is based on a relative slope of thetemperature (T_(N)) vs. time (t) curve for the N^(th) fan unit comparedto an average temperature (T_(ave)) vs. time (t) curve. In someexamples, the method includes dynamically adjusting heaters of therespective fan units including sending a single servo control signal. Insome examples, the sum of all of the PWM control signals for each fan ismaintained at a threshold value.

An example drying assembly has at least 2 fan units where each fan unithas a fan. The fan speed of each fan is adjusted independently tocontrol the air temperature from the fan. The airflow through all of thefans is maintained at a constant value.

An example apparatus includes a dryer; a print engine to accessinstructions to cause a printer to perform a printing operation onmedia; and a processor to control the dryer to force air onto the mediaduring the printing operation, the processor to access and monitortemperature values at respective fans of the dryer, to maintain asubstantially uniform temperature across a width of the media during theprinting operation, and to maintain a sum of air flow exiting the fansto be at a substantially constant value, the processor to at least oneof (1) dynamically adjust at least one of the fans or (2) dynamicallyadjust a heater of the dryer.

In some examples, the print engine is to deposit printing fluid onto themedia. In some examples, based on the temperature values, the processoris to determine an average temperature exiting the fans, when adifference between the average temperature and a temperature value ofthe respective one of the fans satisfies a threshold, the processor toadjust a speed of a respective one of the fans. In some examples, theprocessor is to dynamically adjust at least one of the fans at leastonce per second. In some examples, the processor is to at least one of(1) independently adjust at least one of the fans or (2) independentlyadjust the heater.

An example apparatus includes a print bar to print on media; a dryer toforce air onto the media, the dryer including a first fan associatedwith a first heater and a second fan associated with a second heater;and a processor to access and monitor temperature values at the firstand second fans, to maintain a sum of air flow exiting the first andsecond fans at a substantially constant value, and to at least one of(1) dynamically adjust at least one of the first fan or the second fanor (2) dynamically adjust at least one of the first heater or the secondheater to maintain a substantially uniform temperature across a width.

In some examples, based on the temperature values, the processor is todetermine an average temperature exiting the first and second fans, whena difference between the average temperature and the temperature valuesof a respective one of the first and second fans is below a threshold,the processor to adjust a speed of the respective one of the first andsecond fans. In some examples, the processor is to access thetemperature values approximately every second. In some examples, theapparatus includes a first temperature sensor associated with the firstfan and a second temperature sensor associated with the second fan, thefirst and second temperature sensors to measure the temperature values.In some examples, the dryer includes a support to which the first fanand the second fan are coupled. In some examples, the first fan isspaced longitudinally spaced away from the second fan on the support.

In some examples, the apparatus includes a first housing in which thefirst fan and the first heater are disposed and a second housing inwhich the second fan and the second heater are disposed. In someexamples, the processor is to control the first heater and the secondheater using a single signal. In some examples, the processor is to atleast one of (1) independently adjust the first fan or the second fan or(2) independently adjust the first heater or the second heater.

An example computer readable medium comprising instructions that, whenexecuted, cause a machine to at least: force air onto media during aprinting operation with fans, the fans respectively including a fan, aheater, and a temperature sensor; obtain temperature values from thetemperature sensors, the temperature values representing a temperatureof air exiting the respective one of the fans; and dynamically adjust atleast one of (1) at least one of the fans or (2) at least one of theheaters to maintain a substantially uniform temperature across a widthof the media during the printing operation and to maintain a sum of airflow exiting the fans to be at a substantially constant value.

In some examples, the printing operation includes depositing printingfluid onto the media. In some examples, the instructions cause themachine to, based on the temperature values, determine an averagetemperature exiting the fans, when a difference between the averagetemperature and a temperature value of the respective one of the fanssatisfies a threshold, adjust a speed of the respective one of the fans.In some examples, the instructions cause the machine to at least one of(1) independently adjust at least one of the fans or (2) independentlyadjust at least one of the heaters. In some examples, the instructionscause the machine to dynamically adjust at least one of the fansapproximately every second. In some examples, the instructions cause themachine to control the first heater and the second heater using a singlesignal.

1. An apparatus, comprising: fans; a temperature sensor; a print engineto access instructions to cause a printer to perform a printingoperation on media; and a processor responsive to an output of thetemperature sensor to control the fans to force air onto the media tocause a substantially uniform temperature to be maintained across awidth of the media during the printing operation and to cause a sum ofair flow output by the fans to be maintained at a substantially constantvalue.
 2. The apparatus of claim 1, wherein the processor is to:dynamically adjust the fans; or dynamically adjust a heater.
 3. Theapparatus of claim 2, wherein the processor is to dynamically adjust thefans a rate of once per second or greater.
 4. The apparatus of claim 2,wherein the processor is to: independently adjust the fans; orindependently adjust the heater.
 5. An apparatus, comprising: a printbar to print on media; a first heater; a second heater; a first fanassociated with the first heater; a second fan associated with thesecond heater; and a processor to access and monitor temperature valuesassociated with the first and second fans, to maintain a sum of air flowgenerated by the first and second fans at a substantially constantvalue, and to dynamically adjust (1) the first fan, (2) the second fan,(3) the first heater, or (4) the second heater to maintain asubstantially uniform temperature across a width of the media.
 6. Theapparatus of claim 5, wherein the processor is to access the temperaturevalues approximately every second.
 7. The apparatus of claim 5, furtherincluding a first temperature sensor associated with the first fan and asecond temperature sensor associated with the second fan, the first andsecond temperature sensors to measure the temperature values.
 8. Theapparatus of claim 5, further including a support to which the first fanor the second fan is coupled.
 9. The apparatus of claim 8, wherein thefirst fan is longitudinally spaced away from the second fan on thesupport.
 10. The apparatus of claim 5, further including a first housingin which the first fan and the first heater are disposed and a secondhousing in which the second fan and the second heater are disposed. 11.The apparatus of claim 5, wherein the processor is to control the firstheater and the second heater using a single signal.
 12. The apparatus ofclaim 5, wherein the processor is to: independently adjust the first fanor the second fan; or independently adjust the first heater or thesecond heater.
 13. A computer readable medium comprising instructionsthat, when executed, cause a machine to: obtain temperature valuesassociated with first and second fans; and dynamically adjust at leastone of (1) the first fan, (2) the second fan, (3) a first heaterassociated with the first fan, or (4) a second heater associated withthe second fan to maintain a substantially uniform temperature across awidth of media during a printing operation and to maintain a sum of airflow generated by the fans to be at a substantially constant value. 14.The computer readable medium of claim 13, wherein the printing operationincludes depositing printing fluid onto the media.
 15. The computerreadable medium of claim 13, wherein the instructions cause the machineto: independently adjust the first or second fans; or independentlyadjust the first or second heaters.
 16. The computer readable medium ofclaim 13, wherein the instructions cause the machine to dynamicallyadjust the first or second fans approximately every second.
 17. Thecomputer readable medium of claim 13, wherein the instructions cause themachine to control the first heater and the second heater using a singlesignal.